The present invention relates to a process for the preparation of aqueous solutions of glucans having a β-1,3-glycosidically linked main chain and side groups having a β-1,6-glycosidic bond thereto by fermentation of fungal strains, which secrete said glucans into the fermentation broth, in an aqueous culture medium, the separation of the glucans from the fermentation broth being effected with the use of asymmetrical filter membranes.
In natural mineral oil deposits, mineral oil is present in the cavities of porous reservoir rocks which are closed off from the earth's surface by impermeable covering layers. The cavities may be very fine cavities, capillaries, pores or the like. Fine pore necks can have, for example, a diameter of only about 1 μm. In addition to mineral oil, including natural gas fractions, the deposits comprise water having a higher or lower salt content.
In mineral oil production, a distinction is made between primary, secondary and tertiary production.
In primary production, after sinking of the well into the deposit, the mineral oil flows by itself through the well to the surface owing to the autogenous pressure of the deposit. However, in general only from about 5 to 10% of the amount of mineral oil present in the deposit, depending on the type of deposit, can be extracted by means of primary production, after which the autogenous pressure is no longer sufficient for extraction.
Secondary production is therefore used after the primary production. In secondary production, further wells are drilled into the mineral oil-carrying formation, in addition to the wells which serve for production of the mineral oil, the so-called production wells. Water and/or steam is forced into the deposit through these so-called injection wells in order to maintain or to increase again the pressure. By forcing in the water, the mineral oil is forced slowly through the cavities in the formation, starting from the injection well, in the direction of the production well. However, this functions only as long as the cavities are completely filled with oil and the water pushes the more viscous oil in front of it. As soon as the low-viscosity water penetrates through cavities, it flows from this time on along the path of least resistance, i.e. through the resulting channel between the injection wells and the production wells, and no longer pushes the oil in front of it. As a rule, only from about 30 to 35% of the amount of mineral oil present in the deposit can be extracted by means of primary and secondary production.
It is known that the mineral oil yield can be further increased by tertiary oil production measures. Tertiary mineral oil production includes processes in which suitable chemicals are used as assistants for oil production. These include the so-called “polymer flooding”. In polymer flooding, an aqueous solution of a polymer having a thickening effect is forced instead of water through the injection wells into the mineral oil deposit. By forcing in the polymer solution, the mineral oil is forced through said cavities in the formation, starting from the injection well, in the direction of the production well, and the mineral oil is finally extracted via the production well. Owing to the high viscosity of the polymer solution, which is adapted to the viscosity of the mineral oil, the polymer solution can no longer, or at least not so easily, break through cavities as is the case with pure water.
A multiplicity of different water-soluble polymers have been proposed for polymer flooding, i.e. both synthetic polymers, such as, for example, polyacrylamides or copolymers comprising acrylamide and other monomers and also water-soluble polymers of natural origin.
Suitable thickening polymers for tertiary mineral oil production must meet a number of specific requirements. In addition to sufficient viscosity, the polymers must also be thermally very stable and retain their thickening effect even at high salt concentrations.
An important class of polymers of natural origin for polymer flooding comprises branched homopolysaccharides obtained from glucose. Polysaccharides comprising glucose units are also referred to as glucans. Said branched homopolysaccharides have a main chain of β-1,3-linked glucose units, of which—in statistical terms—about every third unit has a β-1,6-glycosidic linkage to a further glucose unit. Aqueous solutions of such branched homopolysaccharides have advantageous physicochemical properties, so that they are particularly suitable for polymer flooding.
Homopolysaccharides of said structure are secreted by various fungal strains, for example by the Basidiomycetes Schizophyllum commune, which exhibits filamentous growth and, during the growth, secretes homopolysaccharide of said structure having a typical molecular weight Mw of from about 5 to about 25·106 g/mol (trivial name schizophyllan). Homopolysaccharides of said structure which are secreted by Sclerotium rolfsii may furthermore be mentioned (trivial name: scleroglucans).
It is important for polymer flooding that the aqueous polymer solution used for this purpose comprises no gel particles or other small particles at all. Even a small number of particles having dimensions in the micron range may block the fine pores in the mineral oil formation and thus at least complicate or even stop the mineral oil production. Polymers for tertiary mineral oil production should therefore have as small a proportion as possible of gel particles or other small particles.
For use for polymer flooding, it is therefore important that solutions of said homopolysaccharides are substantially free of cells and cell fragments, since these otherwise block the mineral oil formation, which complicates the extraction of the mineral oil or even makes it impossible. The so-called Millipore Filtration Ratio (MPFR value) can be used as a characteristic for a good quality of a polymer solution. The way in which the filter resistance changes in the course of time during filtering of a solution is determined here.